Image display apparatus is used to project visual images for various applications, including entertainment, training, education, public display of information and many others. The image display apparatus may use multiple projectors to produce a single image by the use of two or more projectors producing contiguous images, by the use of two or more projectors projecting partially overlapping images which may be blended to produce an extended continuous image, by the use of a single projector to produce a relatively small image, for example a target image or star field, in a large dark or relatively dark field or in a bright field, or by any other combination of projectors with or without overlap of the projected images.
In such use of projectors it is desirable to achieve consistency in image quality across the total image, and this means controlling at least the color and brightness across the various portions of the resultant image over a wide dynamic range from dark to light. This can be problematic when using projectors with modulators which modulate light, because of the inability of some modulators to achieve extinction, resulting in a finite minimum image brightness, or black level, above true black. When this occurs in image display apparatus employing a single projector, it results in a reduction of the contrast ratio which can be achieved in the resultant image and can result in the production of a `halo` around the projected image. When it occurs in image display apparatus employing multiple projectors, it can additionally result in the generation of artifacts in the display due to differences in the black level in projectors producing contiguous images, or due to the additive effects of the finite black level in areas in which projected image overlap occurs. If the modulator is a spatially addressed modulator which has defects in some modulator elements, there is the additional possibility of a bright or dark spot or spots in the image.
There are various ways of making projectors for image display systems, and these include, but are not restricted to, cathode ray tubes where the phosphor image is projected via a lens to a viewing screen, spatial light modulators such as liquid crystal light valves and digital mirror devices which are addressed electronically and modulate a high intensity light, and light amplifiers such as an image light amplifier which use a low intensity image to modulate a high intensity light.
The light output from a cathode ray tube projector is a function of the phosphor selected and the energy applied by the electron beam to the screen phosphor. The light output from the light modulator, for example a liquid crystal light valve, is a function of the light input to the modulator, as is the light output from an image light amplifier. Other forms of projector are possible. In general, a projector should have a maximum light output, and an instantaneous or modulated light output which is a function of this maximum value. The variation in light output from minimum to maximum which can be achieved is the modulation range of the projector, and results in a dynamic range of the projected image. If the modulator drive signal is digitally processed or generated, the modulation range of the projector will be represented by a number of bits, and the modulator drive signal will be formed into a number of discrete levels, separated by the least significant bit. This will give a shading resolution limited by the lowest bit of the modulating signal as a function of the maximum brightness which can be displayed.
If it is desired to show part of the display at much reduced brightness, as can happen when displaying a night scene, then only part of the modulation range is used, and the number of levels available to modulate the dark parts of the scene is much reduced, limiting the shading resolution which can be achieved in the darker areas of the display and reducing the amount of detail which can be portrayed in the image as the brightness decreases. A similar truncation in the number of levels which can be displayed also arises when the brightness of a scene is varied to represent the time of day, as, for example, may happen when such a projector is used in a flight training simulator during training for flights extending through dusk or dawn.
Some modulators may have minor faults which degrade the total image quality. An example of such a fault in a liquid crystal light valve display panel is a `bright pixel`, that is a pixel, or picture element, which does not respond adequately to a command to reduce the light output. Such a fault will produce a bright pixel or light spot at a fixed location on the display screen.
Some modulators are unable to produce extinction of the projected light, resulting in a black level which is higher than desirable and introducing artifacts into the display.
The current state of the art in image projectors does not permit the display of negative contrast targets, that is where a target is of a lower brightness than the surround, when producing an image by combining the images formed from separate target and background projectors, and this can result in a lack of realism in some training systems.